Piezoelectric energy harvesting converter circuits have been implemented by simply connecting the piezoelectric element to a rectifier and storage capacitor. However, the present applicants found that this direct charge topology converts mechanical energy into electrical energy with a low overall efficiency.
Most electronic circuits that are powered by a piezoelectric energy harvesting converter circuit, such as a microprocessor, operate at much lower voltage than is supplied by the piezoelectric material. The '693 patent describes an improved method of providing a small storage capacitor that is charged to a high voltage by the piezoelectric device and that is generally matched to the impedance of the piezoelectric device. Then charge is transferred via a DC-DC converter into a large storage capacitor, effectively providing a low voltage energy source to power electronic circuits requiring a low voltage.
Suppliers of piezoelectric materials partially addressed the voltage disparity problem by providing piezoelectric materials that deliver a higher current and a lower voltage than was previously available. However, the voltage delivered is still much higher than needed by the electronic circuits and so the DC-DC converter of the '693 application was still advantageous.
The topology of the '693 patent significantly improved efficiency compared to that of the direct charge method alone. However the technique of the '693 patent still limited efficiency because the maximum input voltage that could be applied to the DC-DC converter was around 35 volts, much lower than the 150 volts that was generated by the piezoelectric device. The scheme of the '693 patent also stopped working when available energy was below that required to supply the quiescent current of the DC-DC converter to keep it operating and providing the DC-DC conversion.
U.S. Pat. No. 6,407,483 to Nunuparaov et al (“the '483 patent”) describes an alternate voltage conversion scheme that uses a gas tube and a transformer between the piezoelectric element and the rectifier stage. The gas tube provides a series switch that conducts when the voltage generated by the piezoelectric element reaches a sufficiently high voltage. The threshold for conduction is determined by the type of gas and the spacing of the contacts within the gas tube. Conduction continues for a brief period of time while the piezoelectric element maintains sufficient voltage. A transformer steps down the AC voltage resulting from the impulse in the gas discharge arising from the electricity generated by the piezoelectric element. That lower AC voltage is then rectified and used for powering electronic circuits.
However, gas tubes have a relatively short life with repetitive discharges. A voltage drop during the arc also dissipates a significant amount of the energy provided by the piezoelectric element, lowering efficiency. Transformers introduce additional inefficiency, and are expensive to manufacture. Transformers also are limited to a specified frequency range. In addition, rectifiers introduce a voltage loss which is a more significant fraction of the voltage for a low voltage signal than for a high voltage signal. So locating the rectifiers at the low voltage side of the circuit is less efficient than would be the case with a scheme that provides rectification of the high voltage signal.
The '483 patent also mentions that a thyristor or other semiconductor device can provide the switching in place of a gas discharge tube. The present applicants found, however, that semiconductor thyristors capable of operating at the high voltages provided by piezoelectric devices needed much more current than was available from the high voltage piezoelectric materials.
The circuit with small and large capacitors and the DC-DC converter of the '693 patent was significantly more efficient for powering electronics with a piezoelectric energy harvesting converter than was a direct charge topology. Nevertheless, a better scheme is still needed to provide significantly higher efficiency conversion of mechanical energy into electricity for powering electronic circuits using a piezoelectric energy harvesting converter circuit, and this solution is provided by the following description.